The erection of above-ground structures, particularly in relatively sandy soil, often requires the formation of inground retaining walls for use as a load-bearing foundation or as barrier to prevent the collapse of soil into the excavated area. Where such excavations are made adjacent an existing structure, the retaining wall along the excavation line adjacent the existing structure is necessary to resist soil pressures established beneath the adjoining structure. If no retaining wall is formed, the soil beneath the adjoining structure can collapse outwardly into the excavation and/or damage the existing structure. In addition to retention of soil, retaining walls of this type are often constructed to block the flow of ground water into the excavated area.
A number of methods have been employed to form retaining walls around an excavation site or adjacent structures which adjoin such site. One method has been to employ piles formed of wood or steel which are driven along the excavation line to form the retaining wall. Alternatively, a roll of bored holes are formed along the excavation line which receive reinforced concrete piles to form the retaining wall. Both of these methods produce retaining walls which are not water-tight, and which may require substantial horizontal strengthening to maintain the desired alignment along the excavation line.
Trenching apparatus such as disclosed in Camelleri U.S. Pat. No. 4,843,742 have been proposed as an alternative to the methods and apparatus of forming retaining walls mentioned above. In trenching apparatus of this type, a supporting base capable of being moved in a longitudinal direction along an excavation line carries an elongated trench digger arm supported on one side thereof by a boom structure. The support base is drivingly connected to skids or track assemblies of the type employed in conventional bulldozers which are effective to move the support base and digger arm along the trench line to form a trench of the desired depth. Concrete is poured into the trench immediately behind the moving trenching apparatus, into which appropriate reinforcing bars are inserted, so that an essentially continuously formed, reinforced concrete retaining wall is provided at the excavation site.
One problem with trenching apparatus of the type described in U.S. Pat. No. 4,843,742, particularly in trenching operations performed in sandy soils, involves cave-in of the trench as it is being formed. The boom structure mounted to the support base carries the heavy digger arm alongside the support base, thus creating a substantial force on the skid or track assembly located closest to the trench. It has been found that the pressure exerted by such track assembly can cause the adjacent soil to cave into the trench at the same time or immediately after the trench is dug.
A second problem with trenching apparatus of the type disclosed in U.S. Pat. No. 4,843,742 is instability during a trenching operation. Because the digger arm is heavy, unless the track assemblies mounted on opposite sides of the support base have substantial lateral spacing therebetween, the apparatus tends to tip over. The extent of lateral spacing between the track assemblies is limited, however, because the trenching apparatus must be narrow enough to load onto conventional trailers for transport on roads and highways. There has therefore been a need to provide a trenching apparatus having acceptable stability, but which can also be readily transported on commercially available trailers or other heavy equipment hauling devices.